1. Field of the Invention
The present invention relates to a method and apparatus for controlling the rate of fuel injection in internal combustion engine. More particularly, the invention is concerned with a method and apparatus for controlling the rate of fuel injection in an internal combustion engine having an intake passage, of a comparatively large length and a fuel injector adapted to inject fuel into the intake passage, so that the fuel is mixed with the intake air thereby forming an air-fuel mixture which is then induced into the combustion chambers of the engine.
2. Description of the Prior Art
Modern internal combustion engines of the type described above incorporate electronic fuel injection controllers. The electronic fuel injection controller is adapted to compute the basic fuel injection time duration, i.e. the basic valve opening time, in accordance with data such as, for example, absolute pressure in the intake pipe and engine speed, and to make various correction computations in accordance with the condition of the engine including the warming-up of the engine, transient state and so forth to determine the final fuel injection time duration. In operation, the fuel injector is opened at each one of predetermined crank angles to achieve so-called synchronous injection.
The correction which is conducted in accordance with the state of warming-up of the engine is usually referred to as "warm-up incremental correction". When the cooling water temperature of the engine is below 70.degree. C. for example, the warm-up incremental correction is made by measuring the instant cooling water temperature by multiplying the basic injection time by a warm-up incremental coefficient which is beforehand determined in relation to the cooling water temperature in such a manner that the coefficient value becomes smaller as the engine cooling water temperature gets higher.
On the other hand, a fuel injection control referred to as "warm-up acceleration incremental correction" is conducted when the engine is accelerated during warming-up, in accordance with the following process. The amount of the acceleration is determined, for example, as the amount of change in the intake pressure. A first value correction coefficient is selected in accordance with the measured value of the amount of change in the intake pressure. Then, a second value correction coefficient is determined in accordance with the measured water temperature. The basic injection time duration is corrected using a warm-up acceleration incremental coefficient which is determined on the basis of the first and second value correction coefficients.
When the engine is accelerated quickly between the successive synchronous injections, the response of the engine will be impaired if any fuel injection is not made until the aforementioned synchronous injection is effected. Therefore, in some engines, the fuel injection is made regardless of the crank angle when a need for quick acceleration of the engine is detected. This injection is referred to as "asynchronous injection". The rate of fuel injection during asynchronous injection is determined in accordance with the degree of acceleration of the engine and the cooling water temperature. For instance, asynchronous basic injection time duration, which takes a greater value as the degree of engine acceleration is large, is determined in accordance with the detected degree of engine acceleration and the value of the determined asynchronous injection basic time duration is corrected in view of the cooling water temperature, thereby determining the final asynchronous injection time duration. The correction in view of the cooling water temperature is intended to improve the transient response characteristics of the engine by increasing the fuel injection rate in the cold state of the engine in which the fuel can hardly be evaporated.
In the internal combustion engine of the kind described, the evaporation of fuel depends on the temperature of the wall defining the intake passage between the fuel injector and the combustion chamber. From this point of view, it is preferred that the temperature of the wall surface of the intake passage between the fuel injector and the combustion chamber provide more relevant information as to the basis for various correcting operations, such as the warm-up incremental correction for determining the increment of fuel injection in accordance with the state of warming up of the engine, the determination of the increment for acceleration during warming up by the use of the second coefficient mentioned before, and the temperature compensation in the asynchronous injection. Namely, in the cold state of the engine, the evaporation of the fuel takes place only at a small rate. The increase of the fuel injection in the cold state, therefore, is made to ensure a sufficient amount of fuel to be induced into the engine thereby stabilizing the engine operation. As a matter of fact, however, the evaporation rate of fuel is directly affected by the temperature of the wall surface of the intake passage between the fuel injector and the combustion chamber of the engine. This is the reason why the various correcting operations in relation to temperature should be made on the basis of the temperature of the wall surface of the intake passage.
The second correction coefficient also is incorporated in view of the smaller fuel evaporation rate in the cold state of the engine, than in the normal operating condition of the engine. In the correcting operation making use of the second coefficient, therefore, it is preferable to use the temperature of the intake passage wall as the basis for the correction.
During the warming up of the engine at an extremely low ambient air temperature, the rise of the temperature of the intake passage wall downstream from the fuel injector lags behind the rise of the water temperature for a long period of time, so that the fuel evaporation rate is kept small for a considerably long time. Under such a condition, even when the asynchronous injection is made to cope with a demand for quick engine acceleration, the engine cannot respond to this demand because only a small amount of fuel is induced into the combustion chamber.
Thus, in the known electronic fuel injection controller in which the warm-up incremental correction, warm-up acceleration incremental correction, by the use of the second coefficient, and the asynchronous fuel injection are made on the basis of the cooling water temperature, it is impossible to optimize the rate of fuel supply to the combustion chamber. As a result, the driveability of the engine is possibly impaired, because the temperature rise of the intake passage wall downstream from the fuel injector lags behind the rise of the cooling water temperature for a long period of time, particularly during the warming up of the engine at extremely low ambient air temperature.
To obviate this problem, it has been proposed to circulate the heated cooling water through a riser formed on the outer wall surface of the intake passage to heat up the intake passage wall and, hence, the fuel thereby promoting the evaporation of the fuel. This proposal, however, cannot perfectly eliminate the above-stated problem, particularly when the ambient air temperature is very low.